1. Technical Field
The present invention pertains to the measurement of the oxygen content of high temperature liquids and more particularly to an improved method and apparatus for the continuous monitoring of oxygen in molten metal.
2. Discussion of the Prior Art
Quality standards of modern manufacturing require steel having extremely low concentrations of contaminants such as sulfur and oxygen, typically a few parts per million. Non-metallic inclusions, particularly Al.sub.2 O.sub.3, formed in the liquid steel are a direct consequence of excess oxygen and ultimately appear as surface defects during the rolling and finishing operations. These surface defects are responsible for a significant portion of costly and disruptive rejected or downgraded steel.
A major goal of modern steel makers is improved precision and control over the chemical properties of molten steel. Continuous real-time monitoring of these properties, especially the oxygen content, throughout the molten phase of steel making would be ideal, but has been heretofore unavailable.
Oxygen content (fugacity) sensing in liquid steel is currently performed using a disposable electrolytic zirconia galvanic cell. In general, the expendable oxygen content sensor (more commonly known as a throwaway or disposable oxygen sensor) is constructed of a solid state electrolytic material. Typically oxygen sensors consist of a ceramic solid state electrolyte (&gt;99 percent oxygen ion conductivity) such as zirconia doped with calcia, magnesia or yttria (CaO, MgO or Y.sub.2 O.sub.3). The sensor ceramics are fabricated as closed end tubes or as thin, dense discs. The opposed surfaces are metallized (e.g. with platinum, gold, silver, etc. ) and the open circuit emf across the metal leadouts provides a measure of oxygen content according to the now well known fugacity (Nernst) equation: EQU E(millivolts)=0.0496T(.degree.K.)log [P.sub.o2 (unknown)/P.sub.o2 (reference)]
where P.sub.o2 is the partial pressure (fugacity) of oxygen and T(.degree.K.) is the absolute temperature in degrees Kelvin. Temperature is measured independently, typically by a thermocouple positioned adjacent the sensing portion of the cell either internally or externally. A reference material of known oxygen content can be impressed on one of the surfaces of the sensor disc and the partial pressure of the unknown material is on the other side.
In the case of the disposable measurement device used in liquid steel, a closed-ended zirconia tube is used with a solid reference material of chrome/chrome-oxide packed into the tube interior.
Theoretically nothing has precluded the use of a zirconia-based sensor for continuous oxygen content measurement, but finding practical solutions to several difficult technical problems has posed obstacles. The first such obstacle, the need for simultaneous and continuous temperature measurement, has recently been overcome by the development of ruggedized protective sheath materials and configurations for thermocouples capable of continuous measurement of molten steel processes at temperatures in excess of 3000.degree. F. even in the presence of aggressive slags. This work is exemplified in U.S. Pat. No. 4,721,534 (Phillippi) and No. 4,721,533 (Phillippi et al). Unsolved, however, have been the problems of the susceptibility of zirconia to thermal shock in all but very small tubes and discs and, especially, the eventual depletion (or aggregation) of oxygen ions in the reference material, typically Cr--Cr.sub.2 O.sub.3, over long and continuous periods of operation in low concentration environments (&lt;16 ppm).
In the 1970's efforts were made to develop improved solid electrolyte-based sensors, especially oxygen sensors, for metal melts. It was hoped that stabilization of the electrochemical material and improved physical processing methods for the sensor fabrication would overcome the physical instability and tendency toward thermal fracture of previous sensors. The quartz tube that contained a thin electrolyte disc in one end was discarded and replaced by a tube formed from the electrolyte material itself. Partially stabilized zirconia, ZrO.sub.2 with about 3 wt % of MgO, was formed into a tubular shape, compacted, and sintered to increase density. The resulting sintered electrolyte material comprises two phases, cubic and tetragonal, and has improved mechanical strength and resistance to thermal shock. The tube can be filled with a solid reference electrode
material together with thermocouple and electrical connection scheme as desired, but while this two-phased electrolyte tube can be used as an oxygen sensor in metallurgical melts and is widely used for this purpose today, such devices are still extremely short-lived, being capable of one ten second measurement in actual steel mill environments.
Although the oxygen partial pressure of Cr--Cr.sub.2 O.sub.3 reference material is well characterized as a function of temperature and has been used for over a decade in disposable short-lived zirconia-based oxygen sensors, the ionic transport of oxygen proceeds from the cell interior to the liquid steel and continuous operation at low oxygen concentrations depletes the available oxygen supply by reducing the chrome oxide to elemental chrome metal. Reference material oxygen depletion ultimately produces zero output voltage with the effect of an apparent, but clearly erroneous, increase in steel O.sub.2 content.
Furthermore, if the O.sub.2 levels are higher than approximately 16 ppm, the ion transport is from liquid steel to cell interior, and the conductive chrome metal constituent is eventually oxidized to a saturation point where electrical continuity may be lost and an open circuit can occur with resultant loss of signal altogether. During this process the cell interior can become "flooded" with oxygen ions, thereby increasing the oxygen reference partial pressure and can yield an apparent reduction of steel oxygen content.
The cumulative effect of O.sub.2 ion saturation or depletion of the reference material eventually causes stalling of the ionic transport process and produces "emf values of zero."Misleading oxygen concentration readings then result when the oxygen partial pressure relationship as a function of temperature for Cr--Cr.sub.2 O.sub.3 is substituted in the fugacity, or Nernst equation. Volume constraints prevent simply increasing the total amount of reference material to offset the time-dependent degradation. An alternative approach is to use an electrolyte material with a much higher resistance to ion transport, such as stabilized thoria. Consequently a diminished ion flow rate and increased useful life results, but with a potential loss of sensitivity and increased cost. Another solution to measurement degradation due to cumulative ion transport is to provide a continuous source of fixed oxygen partial pressure gas, for instance an argon and oxygen mixture, fed from an external source through a tube extending through the probe to the reference side of the sensor circuit. Although the cost of the premixed reference gas would be slight, the cost and complexity of the supply apparatus would be high.